Which is the odd one out and why?
10 21 18 61 25
I was helping a Small Person I know with this. I suggested she check her list of times tables to see if any of the numbers weren't in it (having spotted that 61 is prime). Some minutes later the Small Person declared that 61 wasn't in it, so wrote that as the answer.
I saw her again today. The answer the teacher gave was 25, as it was the only number that didn't feature the digit 1. A valid response, just as valid as the Small Person's, which got marked wrongly.
I think we saw primes in year 5, and you'd want a child to be very familiar indeed with the concept before trying a question like this on them without context. For 11+ / Common Entrance or higher levels, Small Person's answer is the best.
If you have to answer 25, then "it's the only square number" is reasonably year-4-friendly and mathematically cogent. "Not featuring the digit 1" is a
really stupid reason, largely because a 1 in the tens column and a 1 in the units column mean different things, so it's counterproductive to suggest to a learner that this is a pattern to be looking for.
To finish off the challenge with successively less plausible answers, 21 is the only Fibonacci number in the set; and 18 is the only abundant number (it's less than the sum of all its proper factors 9+6+3+2+1 = 21)
Ten is just not very special. It's the only number in the set which isn't special in any really mathematical way. That makes it the odd one out, of course.